In electronic parts which are required to have high density and high reliability, as surface treatment of electronic parts which require wire-bonding mounting or soldering mounting, surface treatment with a noble metal having corrosion resistance and excellent electric characteristics is believed to be effective, in particular, gold film plating has played a center role.
However, gold is a material having scarcity value, and thus its price is likely to rise steeply depending upon market quotations. Accordingly, technical development of an alternative metal has attracted attention.
In particular, palladium is inexpensive as compared with gold metal and has been in the spotlight as an alternative metal to reduce a film thickness of a gold plating film.
However, in highly reliable electronic parts in which not only increase of its price but also increase of wiring density have been accelerated in recent years, characteristics, stability and reliability of a palladium plating film have attracted attention.
As an electroless palladium plating solution has heretofore been used in industrial applications, there has been known an electroless palladium plating solution comprising a water-soluble palladium salt, ethylenedianimetetraacetic acid, ethylenediamine, and sodium hypophosphite, as described, for example, in Patent Document 1.
There has also been known an electroless palladium plating solution containing, as essential components, at least one member selected from the group consisting of a palladium compound, ammonia and an amine compound, an organic compound having divalent sulfur, and at least one member selected from the group consisting of a hypophosphorous acid compound and a boron hydride compound (see, for example, Patent Document 2). From these electroless palladium plating solutions, a palladium-phosphorous alloy plating film is obtained.
Besides, there has been known an electroless palladium plating solution containing at least one member selected from the group consisting of a palladium compound, ammonia and an amine compound, and at least one member selected from the group consisting of formic acid, sodium formate and potassium formate (see, for example, Patent Document 3).
The above-mentioned electroless palladium plating solution in Patent Document 1 has not only a drawback of poor storage stability but also a drawback that it is degraded in a short time in an industrial mass production line and thus has a short duration of life as a plating solution. Further, any of plating films obtained from the plating solution are crack-prone and poor in wire-bonding properties and soldering properties, and accordingly, has a difficulty in application to electronic parts.
Moreover, the electroless palladium plating solution disclosed in Patent Document 2 has a drawback that since phosphorus and/or boron derived from the hypophosphorous acid compound and/or the boron compound as a reductive component is contained in a plating film, properties of the palladium plating film markedly change between before and after a heat test.
Furthermore, although the electroless palladium plating solution in Patent Document 3 has excellent storage properties and properties of a palladium plating film are stable and show no substantial difference between before and after a heat test, the plating solution has a technical problem that variations in plating film thickness become considerable as a time period for which the plating solution has been used becomes longer, and thus film thickness control is difficult.
Patent Document 1: Japanese Patent Publication No. Sho46(1971)-026764
Patent Document 2: Japanese Laid-Open Patent Publication No. Sho62(1987)-124280
Patent Document 3: Japanese Patent No. 3035763